Gas heaters are used in many homes to supply space heating and water heating. To improve efficiency, some burners mix gas and air together by a blower which is positioned between the gas and air supply lines. Since such a blower is constantly filled with combustible material, it emits gas odors and can be damaged if the gas ignites accidently. Thorough mixing of air and gas may not occur in compact heating systems which use short pipes. Also, high efficiency heat exchangers used in many burners produces exhaust condensates. Thus, relatively expensive exhaust pipes are needed to remove exhaust gases containing corrosive condensates from the heater.